This invention relates to an inspection device and method for optically inspecting operating holes formed in heads of screws (among screws having a head referred to in this invention, bolts having an operating hole are included) or the heads' top surfaces, and a method for detecting that chipping has developed in a punch of a header for working the heads of screws by use of the inspection device.
Screws with a head are inspected for necessary items after manufacture and delivered after removing defectives. Among inspection items, there is inspection of operating holes formed in heads of screws. Among operating holes of screws, there are hexagonal, Torx, square, cross, plus-minus, which is a combination of cross and minus, HIOS (trade name) and special-shaped holes.
Operating holes are simultaneously formed when screw heads are formed by a header. If there is minute chipping at a tip of a portion of a punch of the header for forming operating holes, it is transferred to a bottom of a hole, so that the bottom will partially bulge. Further, after working of operating holes by the header has been normally performed, foreign matter produced due e.g. to working of threads may enter the hole and the foreign matter may be fixed to and remain in the bottom of the hole when threads have been plated with the foreign matter stuck to the bottom of the hole. In this case too, the bottom of the hole will bulge.
If such defects develop, an operating bit cannot be properly received in the operating hole, so that there may arise trouble in operation of the screw, or the hole may be crushed due to forcible operation. Thus, inspection of such an operating hole has heretofore been performed by inserting a dummy bit to confirm a depth of such a hole.
Inspection by a dummy bit has a problem in terms of a treating capacity, and a cost is high too. Therefore, we thought of employing an efficient image inspection method. But with a conventional image inspection device, only if inspection areas are limited to central portions of operating holes, is inspection is possible. But it was impossible to inspect more important portions where defects tend to develop.
Defects of operating holes tend to develop not at the central portions of the holes but at outer peripheral portions thereof. For example, since the above-described chipping of the header develops at an outer peripheral edge at the tip of the punch, defects of operating holes due to this chipping naturally develop at the outer peripheral portions of the holes.
If trials are made to inspect the outer peripheral portions of the operating holes, where these defects concentrate, with an existing inspection device, a shaded region is formed at edge portions along an outer periphery of the operating holes, so that it is difficult to see a difference between recesses and protrusions of hole bottoms from images of a camera. Since defects are protrusions, it is necessary to see the recesses and protrusions of the hole bottoms for image inspection. Since this was impossible, there was no other way but to employ an inefficient method of confirming the depth with a dummy bit.
Therefore, an object of this invention is to make it possible to detect defects of operating holes with image inspection to improve inspection efficiency.